1. Field of the Invention
The present invention relates to a multi-channel sample and hold circuit that holds a signal sampled or selected from signals of a plurality of channels and a multi-channel A/D converter that uses the multi-channel sample and hold circuit.
2. Description of the Related Art
JP-2007-36580-A discloses an A/D converter that is used in a vehicle engine ECU (electric control unit), body ECU or the like. Such an A/D converter converts analog output signals of various sensors used for engine control and body control, circuit voltage of the ECU's and battery voltage into digital signals. Because the number of the analog signals to be converted may amount to tens or more, it is not practical for an IC chip to form thereon as many A/D converters as the analog signals to be converted because of much increase of the mounting surface area of the IC chip.
As shown in FIG. 17, an IC 101 usually includes a multiplexer 102 and an A/D converter 103. The A/D converter 103 converts a signal that is selected by the multiplexer 102 from signals of a plurality of sensors 104a, 104b, 104c. For this purpose, the A/D converter 103 is allotted to each of three channels Ch1, Ch2 and Ch3 in a time sharing mode.
The A/D converter 103 is connected to the multiplexer 102 by a common line 105. The multiplexer 102 is constructed of analog switches 111, 121, 131, and the A/D converter 103 includes a sample and hold circuit 106. The analog switches 111, 121, 131 are respectively connected to sensors 104a, 104b, 104c via signal terminals 110, 120, 130 and the channels Ch1, Ch2, Ch3.
Incidentally, there is a sensor that has a high output impedance. For example, a knock sensor, which is made of piezoelectric ceramics, has an output impedance of more than 100 kΩ. Usually, resistors Rex1, Rex2, Rex3 are respectively inserted between the sensors 104a, 104b, 104c and the signal terminals 110, 120, 130 so as to prevent surge noises from getting into the IC 101 via a wire harness connecting the sensors 104a, 104b, 104c with an ECU.
Further, because the common line 105 of the multiplexer 102 is connected with analog switches 111, 121, 131, the common line 105 has to have a sufficient capacity to feed the transistors of the analog switches 111, 121, 131. Besides, the common line 105 increases its length as the number of the channels Ch1, Ch2, Ch3 increases. That is, the common line 105 increases its stray capacitance Ccom, which may amount to 5 pf. On the other hand, there are stray capacitances of several pf between each of the signal terminals 110, 120 130 and the ground.
The stray capacitance Ccom is charged by the line voltage of the common line 105 when an A/D conversion is carried out through one of the channels Ch1, Ch2, Ch3. If the output impedance of the sensor is very high, the stray capacitance Ccom is not fully charged in a sampling time, resulting in an error of A/D conversion.
This error can be prevented if external capacitors Cex1, Cex2, Cex3 are connected between the channels Ch1, Ch2, Ch3 and the ground to add capacitance to stray capacitances Ct1, Ct2, Ct3 of the signal terminals 110, 120, 130, thereby increasing the capacitance of the signal terminals 110, 120, 130. However, as the number of the channels increases, the number of the external capacitors increases, so that the mounting surface area+ of a printed board of the ECU increases.
JP-2002-111461-A discloses a circuit for pre-charging the common line that can prevent the above-stated problem. However, it is necessary to provide a voltage follower for each channel, which increases the mounting surface of the IC chip.